The trauma patient may experience pain from a variety of sources. Many, such as multiple trauma, superficial musculo-skeletal injuries, fractures, abrasions and degloving injuries are readily obvious. However deep musculo-skeletal injuries, neuropathic pain, thrombosis and ischaemia, multiple cannulae, thoracic drains, repeated injections and a distended bladder may not be apparent as a source of pain.

Disadvantages of Pain in the Trauma Patient

There are many disadvantages of pain documented in human trauma patients. These include: increased stress response with resultant increased metabolic and energy demands; delayed wound healing; reduced immune function; weight loss; lack of sleep; decreased mobility, with an increase in conditions associated with prolonged recumbency (e.g., pneumonia, nosocomial infections). Adequate pain control has been shown to result in shorter hospital stay and a more rapid return to normal function.

Signs of Pain in the Trauma Patient

Trauma and debilitation may obscure the usual behaviour associated with pain. In addition, in the trauma patient, recognition of pain may be complicated by agitation. If in doubt about the presence of pain, short acting analgesics should be administered. Signs indicative of pain include: restlessness (shifting position, reluctance to lie down); change in demeanour especially interactive behaviour; vocalising: grunting, whining, crying or howling (animals that vocalise are more likely to receive analgesia, however absence of vocalisation does not mean that an animal is not in pain); excessive licking or mutilation; shaking, shivering, salivation, ears flat against head; dilated pupils; and star gazing. In a hospital setting, changes in heart rate, respiratory rate and blood pressure are only of use if considered in association with the above factors.

Which Analgesic Drug?

Analgesic drugs fall into three main groups: 1) opioids, 2) non-steroidal anti-inflammatory drugs (NSAIDs), and 3) local anaesthetics. In addition, ketamine (and other NMDA antagonists), medetomidine, tramadol, gabapentin and tricyclic antidepressants have analgesic properties, which may be beneficial, especially when treating chronic pain. A balanced or multi-modal approach to analgesia provides the optimum pain relief, particularly in the trauma patient.

The most valuable of the above groups for treating acute pain is the opioids, of which the pure mu agonists morphine, pethidine and fentanyl are the most readily available and useful (Table 1).

Morphine is a potent analgesic that may be administered by the intramuscular, intravenous (over 2-3 minutes, or by continuous infusion), epidural, intra-articular and ocular routes. This is a controlled drug so has strict regulations over storage, use, disposal etc. In spite of this inconvenience, its versatility makes it the drug of choice in many trauma patients. Vomition and bradycardia, which may be seen in the healthy ambulatory patient, are rarely a problem in the trauma patient.

Pethidine (meperidine) is a useful short-acting opioid; analgesia seldom lasts longer than 90 minutes. If in doubt about whether a patient requires analgesia, pethidine may be used as onset of action is usually less than ten minutes. As it does not cause vomition, it is an alternative to morphine; it also results in less sedation. Do not administer IV, as it causes histamine release.

Fentanyl has a rapid onset (2-3 minutes) and short duration of action (30 min approx). It can therefore be titrated to effect by diluting 2-5 µg kg-1 and administering until the animal is calm. High doses and CRI (continuous rate effusion) cause respiratory depression and are best avoided. Transdermal patches are suitable for moderate pain only.

Buprenorphine has a long onset of action (30 min) and lasts 6-8 hours. Due to its high binding affinity for the mu receptor, buprenorphine may prevent the full action of transdermal fentanyl and epidural morphine. Buprenorphine is suitable for moderate pain in dogs, but may be more suited for severe pain in cats.

Butorphanol has a similar duration of action to pethidine, but with more sedation and less analgesia. As it is an antagonist at mu receptors it should not be combined with morphine, pethidine or fentanyl.

Table 1. Opioids in common use in veterinary practice.

Drug

Route

Dose

Notes

Morphine

IM

0.2-0.5 mg kg-1

Additional increments if necessary

IV

0.1-0.2 mg kg-1

Must be given very slowly (over 2-3 minutes)

IV CRI

0.1 mg kg-1 h-1

Dysphoria after 16-20h

Epidural

0.1 mg kg-1

Urinary retention likely

Pethidine

IM

3-5 mg kg-1

Do not administer IV

Fentanyl

IV

2-5 µg kg-1

Give slowly to effect

Trans-dermal

2-4 µg kg-1 h-1

Inadequate analgesia for severe pain

Buprenorphine

IV or IM

10-20 µg kg-1

Inadequate analgesia for severe pain (dog)

Butorphanol

IV or IM

0.2-0.5 mg kg-1

Inadequate analgesia for severe pain

There are many NSAIDs which provide excellent analgesia in veterinary patients. The trauma patient is more prone to the toxic effects of these drugs and their use should be avoided until after adequate fluid resuscitation and assessment of renal and gastro-intestinal function. They must NOT be used concurrently with steroids. The dose and dosing interval is usually different in cats (Lascelles et al., 2007).

Local anaesthetics (lidocaine and bupivacaine) completely block nociception and may be used in a variety of ways. These include lumbo-sacral epidural injection (± catheter), intercostal nerve blocks, intrapleural instillation, wound catheters and splash blocks on wounds and fractures. Lidocaine has also been used IV, on its own and with morphine and ketamine to provide analgesia. Cats are sensitive to the toxic effects of lidocaine and so it is best avoided.

Useful Techniques for Administration of Analgesics

Morphine Continuous Rate Infusion (CRI)

This provides good background analgesia (with sedation) after major trauma or surgery, especially when NSAIDs may not be administered. Additional incremental doses of 0.1-0.2mg kg-1 may be required to treat 'break-through' pain.

Morphine by Epidural Injection

Lumbo-sacral epidural injection of morphine provides 12 to 22 hours of analgesia without the motor blockade associated with local anaesthetics. Analgesia is supplied to the pelvic limbs, abdomen and may even extend to the thorax. Epidural catheters suitable for dogs >10kg are available (Perifix 401 Filter Set, B Braun 18G). Provided these are introduced aseptically and maintained appropriately, they may be used for intermittent injections or continuous rate infusions for many days (Hansen, 2001). When topping up morphine, we have found it useful to inject a small dose of lidocaine first as this will prevent vomition; always inject drugs slowly into an epidural catheter.

Fentanyl Patches (Durogesic)

Patches are available in four sizes which release 25, 50, 75 and 100µg h-1 for 72 h. In dogs it may take up to 24h (12h in cats) to reach peak plasma levels so they should be applied as soon as possible to the trauma patient. In the interim, analgesia must be supplied by other agents. Additionally there is large inter-animal variation in plasma levels and so they are seldom adequate as a sole analgesic agent. They should be applied to a clipped area of skin and covered with an occlusive dressing. They must be disposed of carefully as there are concerns regarding human abuse and accidental ingestion. The 25µg h-1 patch should be partly covered when used in small dogs and cats.

Ketamine Infusion

Ketamine's mode of action is unique among the analgesic agents; it acts at NMDA receptors and can block central sensitisation and 'wind-up'. For this reason, it is especially useful in patients with chronic pain or those who fail to respond to conventional analgesic therapy. Dose: loading dose: 0.2-0.5mg kg-1 IV followed by a CRI of 1-2µg kg-1 h-1. Ketamine is best avoided in patients with head injury as it may increase intra-cranial pressure.

Lidocaine Infusion

Low dose lidocaine CRI may induce mild sedation and augment opioid analgesia in dogs. Infusions are generally reserved for patients that require high doses of opioids to control pain. Doses used generally do not cause cardiovascular depression; however side effects may be seen after prolonged use. Dose: 0.25-2.0mg kg-1 IV followed by CRI 0.5-2mg kg-1 h-1. Not for cats.

MLK Infusions

Add morphine (60mg), lidocaine (500mg) and ketamine (60mg) to 500ml of fluids and administer at 1ml kg-1 h-1. This provides excellent background analgesia (with sedation) which can be tapered to the patient's needs. Lidocaine should be omitted in cats.

Medetomidine Infusions

Low doses of medetomidine provide good analgesia, especially if used as an adjunct to other techniques, and do not have the profound cardio-vascular side effects associated with pre-medication. Loading dose: 1-2µg kg-1 followed by CRI 1-3µg kg-1 h-1.

Sedation with low dose acepromazine (0.01-0.02mg kg-1 IM) or a benzodiazepine (midazolam 0.1-0.2mg kg-1 IM) may be necessary if agitation continues in spite of repeated doses of analgesics. Injured patients should not be sedated without the benefit of analgesia.

Main Contraindications to the Use of Analgesics

Analgesia should not be administered until the patient has been evaluated and emergency cardiovascular stabilisation has commenced (IV fluids, oxygen etc).

Mentation should be assessed prior to administration of drugs that will cause sedation--this includes all analgesics except NSAIDs and local anaesthetic techniques.

High does of opioids may cause respiratory depression and retention of carbon dioxide, resulting in increased cerebral blood flow and raised intracranial pressure. High doses are thus best avoided in patients with head trauma, unless ventilatory support is provided.

Morphine may cause vomition in ambulatory patients. This is not desirable in certain circumstances, e.g., penetrating eye injury.

NSAIDs have many potential side effects--reduction in renal autoregulation, gastric mucosal protection and platelet activity. While newer NSAIDs (carprofen and meloxicam) have a lower incidence of these problems they are still a possibility in the presence of haemorrhage and reduced renal and gastro-intestinal blood flow. In the trauma patient, NSAIDs should not be used unless adequate fluid resuscitation has been carried out and never in conjunction with steroids.

Ketamine has the potential to increase cerebral blood flow (and thus intra-cranial pressure) and is best avoided in patients with head trauma.

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